1. Field of the Invention
The present invention is directed, in general, to wireless communication systems and, more specifically, to the adaptation of transmission power adjustments in Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication systems.
2. Description of the Related Art
The invention considers transmission power adjustments in SC-FDMA communication systems and is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE). The invention assumes the UpLink (UL) communication corresponding to the signal transmission from mobile User Equipments (UEs) to a serving base station (Node B). A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a base transceiver system (BTS), an access point, or some other terminology.
The UL physical channel carrying the transmission of data information signals from UEs is referred to as the Physical Uplink Shared CHannel (PUSCH). In addition to data information signals, the PUSCH carries the transmission of Reference Signals (RS), also known as pilot signals. The PUSCH may also carry the transmission of Uplink Control Information (UCI) signals. The UCI signals may include any combination of positive or negative acknowledgement signals (ACK or NAK, respectively), Channel Quality Indicator (CQI) signals, Precoding Matrix Indicator (PMI) signals, and Rank Indicator (RI) signals. In the absence of data information signals, UCI signal transmission is through the Physical Uplink Control CHannel (PUCCH).
The ACK or NAK signal is associated with the use of Hybrid Automatic Repeat reQuest (HARQ) and is in response to the respective correct or incorrect data packet reception in the DownLink (DL) of the communication system corresponding to signal transmission from the serving Node B to a UE. The CQI signal from a UE is intended to inform the serving Node B of the channel conditions the UE experiences for signal reception in the DL, enabling the Node B to perform channel-dependent scheduling of DL data packets. The PMI/RI signals from a UE are intended to inform the serving Node B how to combine the transmission of a signal to the UE from multiple Node B antennas in accordance with the Multiple-Input Multiple-Output (MIMO) principle. Any one of the possible combinations of ACK/NAK, CQI, PMI, and RI signals may be transmitted by a UE in the same Transmission Time Interval (TTI) with data signal transmission or in a separate TTI without data transmission.
UEs are assumed to transmit UCI and/or data signals in the PUSCH over a TTI corresponding to a sub-frame.
FIG. 1 illustrates the sub-frame structure assumed in the exemplary embodiment of the invention. The sub-frame 110 includes two slots. Each slot 120 further includes NsymbUL=7 transmission symbols, for example, and each transmission symbol 130 further includes a cyclic prefix (CP) for mitigating interference due to channel propagation effects. The PUSCH transmission in the two slots may be in the same part or it may be at two different parts of the operating BandWidth (BW). RS are transmitted in the middle transmission symbol of each slot 140 and in the same BW as the data signal. The RS are primarily used to provide channel estimation enabling coherent demodulation of the UCI or data signal (DM RS). The PUSCH transmission BW includes frequency resource units, which may be referred to as Resource Blocks (RBs). In an exemplary embodiment, each RB includes NscRB=12 Resource Elements (REs), or sub-carriers, and a UE is allocated MPUSCH consecutive RBs 150 for its PUSCH transmission.
In order for a Node B to determine the RBs where to schedule PUSCH transmissions and the associated Modulation and Coding Scheme (MCS), a CQI estimate, such as a Signal-to-Interference and Noise Ratio (SINR) estimate, is needed over the UL scheduling BW. Typically, this UL CQI estimate is obtained through the separate transmission of an RS sounding the UL scheduling BW (Sounding RS, or SRS). The SRS is transmitted from a UE in a transmission symbol 160 of some UL sub-frames, over MSRS consecutive RBs 170, replacing the transmission of data or control information from the same or from a different UE. The SRS may be transmitted from UEs not having PUSCH transmission in the reference sub-frame and its transmission power is independent of the PUSCH transmission power.
An exemplary block diagram of the transmitter functions for SC-FDMA signaling is illustrated in FIG. 2. Coded CQI bits and/or PMI bits 205 and coded data bits 210 are multiplexed 220 through rate matching. If ACK/NAK bits are also multiplexed, coded data bits are punctured to accommodate ACK/NAK bits 230. The Discrete Fourier Transform (DFT) of the combined data bits and UCI bits is then obtained 240, the sub-carriers 250 for the assigned transmission BW are selected 255, the Inverse Fast Fourier Transform (IFFT) is performed 260, the CP 270 is inserted, and the selection 280 of the power amplification level 285 is applied to the transmitted signal 290. For brevity, additional transmitter circuitry, such as digital-to-analog converter, analog filters, and transmitter antennas are not shown. Also, the encoding process for data bits and CQI and/or PMI bits, as well as the modulation process, are omitted for brevity.
At the receiver, the reverse (complementary) transmitter operations are performed. This is conceptually illustrated in FIG. 3 where the reverse operations of those illustrated in FIG. 2 are performed. After an antenna receives the radio-frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters), which are not shown for brevity, the digital received signal 310 has its CP removed 320. Subsequently, the receiver unit applies an FFT 330, selects 345 the sub-carriers 340 used by the transmitter, applies an Inverse DFT (IDFT) 350, extracts the ACK/NAK bits and places respective erasures for the data bits 360, and de-multiplexes 370 the data bits 380 and CQI/PMI bits 390. As for the transmitter, well known receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity.
Initial transmission of a Transport Block (TB) from a UE may be configured by the Node B through the transmission of a Scheduling Assignment (SA) or through higher layer signaling to the reference UE. In the former case, the PUSCH transmission parameters, including the transmission power and the Modulation and Coding Scheme (MCS) may be adapted by the SA and the respective PUSCH transmission is referred to as adaptive.
PUSCH HARQ retransmissions of a TB are assumed to be synchronous. If the initial transmission occurs in UL sub-frame n, a retransmission will occur at UL sub-frame n+M, where M is a known integer such as, for example, M=8. It is assumed that the Node B transmits to the reference UE a NAK signal in a DL sub-frame n+L prior to the n+M UL sub-frame, where L is a known integer with L<M (for example, L=4).
PUSCH HARQ retransmissions may be adaptive (configured by a SA) or non-adaptive (occurring without SA). Non-adaptive HARQ retransmissions use the same parameters as the initial transmission for the same TB. In particular, the transmission power adjustment and the PUSCH RBs are the same as the ones used for the initial transmission of the same TB.
When UCI or SRS is multiplexed in the PUSCH, it is possible that either the initial transmission includes UCI or SRS while the HARQ retransmission does not, or the reverse. In either case, for non-adaptive HARQ retransmissions, the amount of REs available for data information is different between the initial transmission and the HARQ retransmission for the same TB since the number of RBs remains the same. The UE autonomously performs rate matching or puncturing of coded data symbols to accommodate UCI or SRS transmission. However, the PUSCH transmission power remains the same. This results in a different BLock Error Rate (BLER) for the data between the initial transmission and a HARQ retransmission for the same TB due to the different effective data coding rate. The same may also occur for PUSCH transmissions configured by higher layer signaling (non-adaptive).
For example, a transmission power adjustment ΔTF in sub-frame i may be computed as ΔTF(i)=10 log10(2K(TBS/NRE)−1) where K is a constant, TBS is the TB Size, and NRE=2·MPUSCH·NscRB·(NsymbUL−1) is the number of REs per sub-frame excluding REs used for DM RS transmission (one transmission symbol per slot is used for DM RS transmission in the exemplary structure in FIG. 1). Therefore, the PUSCH transmission power remains the same in non-adaptive HARQ retransmissions regardless of the presence or absence of UCI or SRS.
The data BLER difference between the initial transmission and a non-adaptive HARQ retransmission for the same TB is not deterministic and depends on the difference in the REs available for the data information (difference in data coding rate). If more REs are allocated to data information in the initial transmission (for example, there is no UCI or SRS transmission) than in a HARQ retransmission (for example, there is UCI or SRS transmission), the puncturing or rate matching of data information needed in the HARQ retransmission results in increased data BLER. The reverse applies if fewer REs are allocated to data information in the initial transmission and the data BLER in the non-adaptive HARQ retransmission may be lower than needed for achieving the desired system throughput.
When the previously described variability exists in the number of REs available for data information in the initial transmission and in a non-adaptive HARQ retransmission for the same TB, or for transmissions configured by higher layer signaling, the conventional approach considers that the PUSCH power adjustment in a HARQ retransmission remains the same as in the initial transmission. For example, as it was previously described, ΔTF(i) does not account for UCI or SRS transmission. Then, two distinct cases exist:
(A) UCI or SRS is not included in the initial transmission but is included in a non-adaptive HARQ retransmission. Then, to decode the data, the Node B receiver may insert erasures in REs where UCI or SRS replaces data or account for the different rate matching performed at the UE. The data BLER increases, due to the increase in the effective coding rate that is not compensated by a respective increase in transmission power, and the system throughput decreases.
(B) UCI or SRS is included in the initial transmission but is not included in a non-adaptive HARQ retransmission. Then, during the HARQ retransmission, the UE may transmit additional bits in the REs where UCI or SRS were located in the initial transmission, thereby decreasing the effective code rate with rate matching. The data BLER unnecessarily decreases, due to the decrease in the effective code rate that is not compensated by a respective decrease in transmission power, and the interference management is sub-optimal due to the unnecessarily high transmission power.
Therefore, there is a need to adjust the transmission power depending on the amount of control signals or reference signals in a PUSCH transmission.
There is another need to determine the appropriate adaptation of the transmission power adjustment depending on the amount of control signals or reference signals in a PUSCH transmission.